1. Field of the Invention
The present invention relates to an optical multi/demultiplexer capable of combining and separating signal lightwaves when used in a wavelength division multiplexing (WDM) transmission system that transmits light carrying a plurality of signals each having a different wavelength over an optical fiber transmission line.
2. Description of the Background Art
In recent years, the highly information-oriented society has been urging researchers and engineers to utilize an optical fiber transmission line for large-capacity communication, such as image transmission, and long-distance communication, such as international communication. Recent rapid increase in communication demands through the Internet has been advancing the development and introduction of the WDM transmission system.
The types of WDM transmission systems includes a dense WDM (DWDM) system, which has a narrow wavelength band for signal transmission, and a coarse WDM (CWDM) system, which has a comparatively broad wavelength band. FIG. 7A is a diagram showing an example of a wavelength grid of the signal lightwave used in the DWDM transmission system. FIG. 7B is a diagram showing an example of a wavelength grid of the signal lightwave used in the CWDM transmission system. In the DWDM transmission system, the spacing Δλ between the adjacent wavelengths is as narrow as 0.8 nm. On the other hand, in the CWDM transmission system, the spacing Δλ is as broad as 20 nm, and the entire wavelength band for signal transmission is at least 50 nm. The DWDM transmission system is used for high-speed, large-capacity communication over a long-distance trunk line. The CWDM transmission system is used for communication with comparatively low information density such as that over an access line between a telephone company central office and a customer's premises in an urban area.
The WDM transmission system sometimes requires to combine or separate signal lightwaves carrying a plurality of signals each having a different wavelength for the transmission over an optical transmission line. The types of planar waveguide-type optical circuits capable of combining and separating signal lightwaves include an arrayed waveguide grating (hereinafter referred to as an AWG)-type optical circuit and a Mach-Zehnder interferometer (hereinafter referred to as an MZI)-type optical circuit.
B. H. Verbeek et al. have reported an optical multiplexer incorporating an MZI-type optical circuit in a paper entitled “Integrated four-channel Mach-Zehnder multi/demultiplexer fabricated with phosphorous doped SiO2 waveguides on Si,” which is included in the Journal of lightwave technology, Vol. 6, No. 6, p. 1011 (1988). The optical multiplexer combines four signal lightwaves with an optical circuit in which an optical-circuit stage having one asymmetrical MZI and another optical-circuit stage having two asymmetrical MZIs are cascaded.
In such an optical multiplexer, when the number of signal lightwaves to be combined is increased, the size of the optical circuit inevitably increases. To combine eight signal lightwaves, yet another optical-circuit stage having four MZIs must be additionally cascaded (the total number of MZIs is seven). In the optical multiplexer, the spectral line shape of the loss performance is comparatively narrow after the signal lightwaves are combined. As a result, the multiplexing performance tends to be affected by the shift of the center wavelength due to the variations in the production of the optical circuit and by the fluctuation of the center wavelength due to temperature change.
Manabu Oguma et al. have reported an optical multi/demultiplexer in which MZIs are cascaded in two stages in a paper entitled “Flattened pass-band filter of double cascaded Mach-Zehnder interferometers,” which is included in the proceedings of the 1999 Electronics Society Conference of IEICE (The Institute of Electronics, Information and Communication Engineers of Japan)(p. 202, number C-3-96). When an optical multi/demultiplexer incorporating a single-stage MZI is used, the spectral line shape of the loss performance shows a Gaussian distribution. Consequently, fluctuation of the wavelength of the signal lightwave varies the multiplexing-and-demultiplexing performance. To cope with this problem, the optical multi/demultiplexer reported by Manabu Oguma et al. uses MZIs cascaded in two stages. This structure flattens the spectral line shape in the vicinity of the minimum value of the loss performance to suppress the variations of the multiplexing-and-demultiplexing performance.
Conventional optical multi/demultiplexers are intended to use mainly in the DWDM transmission system. Consequently, optical multi/demultiplexers have mainly been developed for the use in a comparatively narrow wavelength band, such as the C-band (1530 to 1565 nm) or the L-band (1570 to 1610 nm). However, it is considered that optical multi/demultiplexers will also be applied to communications systems with relatively low information density by using the CWDM transmission system. Accordingly, it is required to develop an optical multi/demultiplexer suitable for this application.
In the CWDM transmission system, the wavelength spacing between the adjacent channels in the wavelength band is broad. Therefore, even when the oscillating wavelength of a laser or the center wavelength of each channel to be combined or separated by an optical multi/demultiplexer is shifted to a certain extent due to temperature change or another cause, this shift can be tolerated in this system. This advantage eliminates the temperature controller for maintaining the temperature constant, and widens the specified allowable limits of the wavelength. As a result, low-cost optical components can be used. On the other hand, because the entire wavelength band for signal transmission is broad, the difference in the multiplexing-and-demultiplexing performance between channels tends to be increased.
For example, in the case of an optical multi/demultiplexer employing the AWG-type optical circuit, the depth of focus of the slab-type waveguide differs with the wavelength. This difference causes the difference in the multiplexing-and-demultiplexing performance between channels. With an optical multi/demultiplexer employing the MZI-type optical circuit, the optical coupling ratio at the directional coupler of the MZI differs with the wavelength. Consequently, as the wavelength of the signal lightwave deviates from the designed center wavelength, the multiplexing-and-demultiplexing performance deteriorates and falls away from the desired performance.